Investigation of the ˙NH2 radical in aqueous solution up to 200 °C by pulse radiolysis

Abstract

The absorption spectrum of ˙NH₂ has been measured in aqueous solutions of ammonia at pH 10.3 and ambient temperature. It exhibits two bands with λ_max at 530 nm, ε₅₃₀ = 8.6 ± 0.1 mol⁻¹, and ca. 230 nm, ε₂₃₀ = 23.2 ± 1.2 mol⁻¹. ˙NH₂ is produced in the reaction ˙OH + NH₃(aq) → ˙NH₂ + H₂O with k = (8.0 ± 1.3) × 10⁷ d mol⁻¹ s⁻¹ at 25 °C and E_act = 11.4 ± 0.5 kJ mol⁻¹. The self-reaction of ˙NH₂ occurs with 2k = (3.6 ± 0.6) × 10⁹ d mol⁻¹ s⁻¹ at 25 °C and E_act = 11.6 ± 0.5 kJ mol⁻¹. This temperature dependence indicates that the reaction rate is influenced by the rate of diffusion, and the relatively low value of 2k (25 °C) is rationalised by including a spin statistical factor of 0.25 in the expression for the rate constant. The reaction ˙NH₂ + N₂H₄ has k = (1.18 ± 0.03) × 10⁷ d mol⁻¹ s⁻¹ at 18.5 °C and E_act = 8.2 ± 0.7 kJ mol⁻¹. Some implications of these data for the coolant chemistry of water-cooled nuclear power reactors are noted.